The recent recovery of Mycobacterium paratuberculosis from tissues of patients with Crohn's disease has highlighted the possible etiologic role of this microorganism in the disease. However, the immunological evidence generated by various groups supporting this hypothesis is as yet inconclusive. A specific antibody response might be masked in these patients by the wide antigenic homologies prevailing within the genus Mycobacterium. The present study was undertaken with the purpose of exploring the humoral response to M. paratuberculosis in patients with Crohn's disease, by means of a cross-absorption procedure recently proposed for unveiling the presence of specific antibodies in bovine paratuberculosis. Antibodies IgG to M. paratuberculosis were investigated by enzyme-linked immunosorbent assay in 90 serum samples from 17 patients with Crohn's disease, 23 patients with ulcerative colitis an 14 with other bowel diseases. Samples from 86 subjects without bowel disease (healthy individuals and patients with tuberculosis, mycobacterioses and fungal diseases) were also included as controls. The specificity of these antibodies was explored by the absorption of sera with an ubiquitous Mycobacterium (M. phlei). The results were compared to those obtained by similar ELISA tests employing M. avium or M. tuberculosis as antigens. A faint humoral response to M. paratuberculosis and M. tuberculosis was detected in patients with Crohn's disease. Cross-absorption with M. phlei did not disclose a specific response nor was an increase in antibody levels detected in patients studied periodically. Sera from patients with ulcerative colitis and other bowel diseases also showed a slight reaction to mycobacteria.(ABSTRACT TRUNCATED AT 250 WORDS)